1. Field of the Invention
The present invention relates to sunscreen compositions and methods of protecting the skin from ultraviolet radiation.
2. Description of the Prior Art
Sunscreens are used in the prevention of sunburns. The region of the sunlight which a compound must absorb in order to be an effective sunscreen is dependent on the intensity and wavelength of the solar radiation, and the body's erythemic response to the light. Insignificant amounts of solar radiation reach the earth's surface below 290 nm. The shorter wavelengths are screened out by the atmosphere. On the other hand, studies on the erythema produced by ultraviolet light on untanned human skin indicate a maximal erythemagenic effect at 254 nm and at 287 nm, separated by a minimum at 280 nm. Only the second maximum is within the solar radiation region. It is possible to use these facts to establish what has been called a "sunburn curve." Plotting the solar radiation curve and the erythema curve on the same graph, a sunburn curve can be constructed by multiplying the ordinates of the erythema curve by those of the solar radiation curve for each wavelength. Doing this, it has been reported that maxima in the sunburn curve appear at 304 nm, or are in the range of 290-326 with a maximum at 308 nm. (see for example Riegelman and Penna, Journal of the Society of Cosmetic Chemists, volume 11, 280-289 (June, 1960)).
In order to be a good sunscreen, a compound must possess high absorption properties at 290-310 nm while superimposing on the sunburn curve.
Normally used sunscreening compounds, such as p-aminobenzoic acid (PABA) or benzophenones meet, to some extent or other, the absorption requirements.
The use of sunscreen compounds which block light in the region 290-310 nm, while effective for preventing sunburn, has the undesirable consequence of blocking the precise radiation wavelength needed to transform 7-dehydrocholesterol, the precursor of vitamin D.sub.3 known to exist in the skin, into vitamin D.sub.3. Under normal radiation conditions, with photolysis at 250-320 nm, 7-dehydrocholesterol undergoes ring opening to previtamin D.sub.3 (1) (Holick, M.F. et al, Biochem.Biophys.Res.Comm., 76: 107-114(1977)): ##STR1##
By a dark, thermally induced sigmatropic shift, the previtamin is then converted to Vitamin D (2): ##STR2##
The availability of vitamin D precursors in the skin and their photo-induced transformation to previtamin D, and then to vitamin D, is an efficient physiological source of, and mechanism for the replenishment of vitamin D. However, when sunscreen compounds which block radiation in the ultraviolet region 250-320 nm are used, this replenishment mechanism is blocked. The result is the possibility of serious vitamin D deficiency which results in a breakdown in blood calcium regulation with concommitant hypocalcemia and bone calcium wasting.
It has been disclosed (Holick, M., talk presented at the May 1979 Meetings of the American Society for Clinical Investigation in Washington, D.C., U.S.A., published in: Transactions of the Association of American Physicians, Vol. xcii, 54-63 (1979) and in "Molecular Endocrinology;" MacIntyre and Szelke, eds; Elsevier/North Holland Biomedical Press (1979), 301-308) that topical application of hydroxylated metabolites of vitamin D or hydroxylated provitamin D compounds to the skin combined with U.V. phototherapy is a method for the sustained administration of vitamin D metabolites to patients who suffer vitamin D metabolic disorders. When the hydroxylated provitamins are applied and irradiated with ultraviolet radiation, they convert to hydroxylated previtamins which then thermally isomerize to the hydroxylated vitamin D. This work is also disclosed in Holick et al, New England J. of Med., 303: 349-354 (1980) and co-pending U.S. Application Serial Number 022,393 filed March 21, 1979, by Holick and Uskokovic at the U.S. Patent and Trademark Office.
Hungarian Pat. No. 102,939 to Pless discloses cosmetic creams containing vitamin D precursors (such as ergosterol) which, when irradiated with ultraviolet rays, are transformed into vitamin D. U.S. Pat. 2,060,228 to Lorenz discloses soap compositions containing sterols, which have previously been irradiated to render the resulting sterols anti-rachitic.
The Holick, Pless and Lorenz disclosures, however, deal solely with the application and administration of (hydroxylated) provitamins or vitamin D in standard transparent carriers comprising fatty acid alcohols, semisolid petroleum hydrocarbons, 1, 2-glycols, emulsifying agents, methyl celluloses, polyoxyethylene glycols, or the like.
Leigh, U.S. Pat. No. 3,981,996, discusses pharmaceutical skin compositions comprising a hydrophobic medium, a water-insoluble powder dispersed therein and a medicament absorbed on the powder. It is suggested that among the medicaments, a sun screening preparation comprising vitamin D can be used.
Any attempt to add vitamin D into a sunscreening composition, such as that of Leigh, however, has to take into account the possibility of uncontrolled increased absorption of vitamin D, with concomittant loss of concentration control, and the appearance of side effects such as vitamin D toxicity leading to hypercalcemic states.
A need therefore, continues to exist for sunscreening compositions possessing high absorption properties in the ultraviolet region particularly around 250-320 nm, while superposing on the sunburn curve, and simultaneously not preventing the undesirable loss of the cutaneous photoproduction of previtamin.